Extreme Heat: Heat exhaustion is a condition whose symptoms may include heavy sweating and a rapid pulse, a result of body overheating. It's one of three heat-related syndromes, with heat cramps being the mildest and heatstroke being the most severe. Causes of heat exhaustion include exposure to high temperatures, particularly when combined with high humidity, and strenuous physical activity. Without prompt treatment, heat exhaustion can lead to heatstroke, a life-threatening condition. Fortunately, heat exhaustion is preventable.